Aluminium base alloy with protective coating



July 18, 1944. G. GAUTHIER 2,354,006

ALUMINUM BAISE ALLOY WITH PROTECTIVE COTING Filed Feb. l, 1941 Patented July 18, 1944 ALUMINIUM BASE ALLOY WITH PROTECTIVE COATING Gaston Gauthier, Chambery,

Savoie, France;

vested in the Alien Propertyy Custodian Application February 1, 1941, Serial In France January 26, 1940 1 Claim.

For several years it' has been endeavoured to use aluminium lalloys containing at the same time magnesium, zinc, copper, nickel, and having the following compositions: y

Per cent Zn Up to 15 Mg Up to 7 Cu Up to Ni Up to 3 Al substantially the remainder cracking corrosion has not in fact been obtained.

I'he applicant has noticed the surprising fact that a layer of plating, constituted by an aluminium alloy mainly containing zinc, ensured the protection; the zinc content must be higher than 0.5%.

Either in order to facilitate the intimate Welding of the layer of plating with the alloy member to be protected, or to modify in a definite direction the diffusion between the layers of the alloy elements 'during the heat treatment, or for any other reason, a layer of metal or alloy, of different composition than that of both alloys, for

instance a layer of aluminium of selected purity, f

may be interposed between the layer of plating and the foundation metal.

In the layer of aluminiumzinc alloy can advantageously be incorporated all the known elements for promoting the obtainment or the maintenance of a ne grain during the manufacture (melting, casting), heat treatments and mechanical distortions, such as small quantities of V, Ti, Zr, Mn, etc. together or separately but in total quantities smaller than 27 The elemennts entering into the composition of the protecting layerabove defined could be the usual metalsi'ound on the market or, if a marked incorrodibility of the layer is to be ensured, use vwill be made of the purest metals, so-called reiined metals, that is to say of very low content in impurities, titratlng more than 99.90%.

The plating layer of alloy of the above specied composition, can be plated on the ingotsor members at any one of the phases of manuture: It can be plated either before any transforming operation, or after the various transforming operations (extrusion, forging, rolling, etc.) or on the semi-finished products between certain transforming operations.

Referring to the drawing, Fig. i shows a sheet of metal with the plating on one side and Fig. 2 with the plating on both sides.

The protection process applies not only to the manufacture of members obtained by mechanical transformation processes (extrusion, forging, rolling, etc.) which utilise either ingots, or seminished products, but also applies to the case of foundry members, that is to say, to cast members used after more or less complex machining (fettling, drilling, milling, etc.).

The plating metal may be case or Welded to the foundation metal by any of the well known casting or welding processes.

The protecting alloy layer of the composition specified can also be secured by atomizing by a known process said protecting metal either at the surface of a cast ingot adapted to be transformed by one of the known means (extrusion, forging, stamping, etc.), or at the surface of members adapted to be used such as they come from casting, or again, at the surface of blanks adapted to be subjected to a complementary transforming operation by a known process (extrusion, forging, rolling, etc), or at the surface of iinshed members obtained by known processes (extrusion, forging, rolling, etc.)

I'he protecting layer of the composition specified can also be produced by an electrolytic process, or by a chemical process such as surface precipitation or cementation, that is to say by a modification of the composition of the supercial part of theingot by the action of salts, metals, alloys, intermetallic compounds or mixtures of these various bodies. These modifications can be effected on the ingots, sand-cast or chill-cast members, finished or semi-finished products.

Said cementatlon can consist, as usual, in increasing the content of the cortical layer in certain elements (Zn, for instance), or in diminishing the content in certain other elements (Mg for instance), or by evaporation (eventually during the heat treatment), or by oxidation (by tempering in a' bath which can be an oxidizingy molten bath such as potassium bichromate), or by any other reaction with a gaseous atmosphere,

a liquid or solid salt, an alloy, or a suitable mixture of these bodies.

The plated members, finished or semi-finished products obtained can be subjected to ameliorating heat treatments suited to the foundation alloy. The dissolving treatment could eventually be prolonged until nearly complete homogenization of the plating layer and the foundation metal, or of the plating layer, the intermediate layer and the foundation metal, or of the intermediate layer and of one of the two other layers, if necessary for ensuring the suitable cohesion of the layers one to the other, for instance in view of important plastic or nonplastic distortions to which the member thus treated is to be subsequently subjected; or again for increasing the average mechanical characteristics of the compound product.

Solely for giving an idea of the eillciency of the process as regards protection against corrosion, by way of example, the following experiment will be cited:

A sheet metal plate 1 mm. thick was prepared with an aluminium base alloy having the compo sition:

Per cent Fe 0.60 Si 0.60 Mg 3.20 Cu 1.90 Zn 5.20 Ni 0.10 Ti 0.20

Itv showed as characteristics, after quenching at 475 C. and annealing to 120 for 24 hours:

Tensile strength=55 kg./mm.2 Elonsation=14.'7%

25 and 40 kg./mm.n and the elongations became so small that it was impossible to measure them. Their micrographic examination shows that' an important 4intercrystalline corrosion had taken place.

A sheet metal plate 1 mm. thick was prepared with the same alloy, but covered on both sides for about 5% of the thickness, with an aluminvis im alloy containing 3% of zinc. Treated in the same conditions as above, this sheet metal plate showed the *following characteristics:

Tensile strength= lig/mm.2 Elongation= 15 After being maintained for three months in a salt mist, one obtained:

Tensile strength=50 kg./mm.2 Elongation=13.5%

up to 15%, minor significant quantities of magnesium up to 7% and copper up to 5% and nickel up to 3%, respectively, an adherent coatving thereon comprising an aluminium base alloy containing a minor quantity of zinc in an yamount of the order of4 0.53.0% but not les@ than 0.5%.

GASTON GAUTHIER. 

